Method of manufacturing a motor having thermostatically controlled or limited space heater

ABSTRACT

Methods of manufacturing an apparatus adapted to be connected to a power source, the apparatus for driving a rotatable component. A motor has a stationary assembly and a rotatable assembly in magnetic coupling relation to the stationary assembly, the rotatable assembly in driving relation to the rotatable component, the stationary assembly including windings adapted to be energized by the power source to produce an electromagnetic field for rotating the rotatable assembly, the windings having a maximum desired operating temperature and a predetermined minimum desired temperature rise. A heater is in heat exchange relationship to an outer surface of the windings, the heater adapted to be connected to the power source at least when the motor windings are not energized to generate heat transferred to the windings to increase the temperature of the windings. An optional thermostat may be connected in series between the heater and the power source. The thermostat limits a maximum temperature of the heater so that the generated heat is generally insufficient to cause overheating of the heater and the heat generated inhibits condensation on the windings during periods when the motor windings are not energized. The heater has a surface area and a power density such that the heat generated by the heater generally does not raise the temperature of the heater above a maximum temperature during periods when the motor windings are not energized and substantially all residual heat has dissipated from the motor.

This is a division of application Ser. No. 08/281,336, filed Jul. 27,1994, now U.S. Pat. No. 5,631,50 .

BACKGROUND OF THE INVENTION

This invention relates generally to motors have heaters which are usedto prevent condensation on the motor windings when the motor is notrunning and, in particular, to a motor having a thermostaticallycontrolled space heater.

It has been recognized that condensation tends to develop on a motorwinding after the motor has increased in temperature during an operatingcycle and the motor cools after the operating cycle. In the prior art,this problem has been partially addressed by space heaters used tomaintain the temperature of the motor winding above ambient temperaturewhen the motor is not running. In particular, the prior art has mountedspace heaters adjacent to the motor windings to generate heat which istransferred to the motor windings. Such space heaters are operated whenthe motor is not running. Due to the high surface temperatures developedby such space heaters, a thermally-insulated layer is positioned betweenthe space heater and the motor winding. Although this tends to inhibitheat transfer to some extent, it prevents direct contact of the spaceheater with the motor winding. Because of the high surface temperatureof the space heater, direct contact between the space heater and themotor winding could damage the motor winding by melting windinginsulation or causing other thermal deterioration of the winding.

For some motor applications in hazardous environments, maximum surfacetemperatures must be limited in order to prevent or minimize thepossibility of auto-ignition of gases, vapors or dust that may bepresent. In such environments, prior art motors with space heaters maynot be used or useful because of the high surface temperatures developedby the space heaters.

Therefore, there is a need for a motor having a space heater with lowmaximum surface temperatures which would prevent thermal damage to themotor windings and which could be used in hazardous locations. There isalso need for such motors which are efficient and reduce the powerrequirements to power the space heater when the motor is not running.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a motor having athermostatically controlled or limited space heater which has a maximumsurface temperature limit allowing it to be employed in hazardousenvironments while minimizing or preventing the possibility ofautoignition of gases, vapors or dust that may be present in suchhazardous environments.

It is another object of this invention to provide a motor having athermostatically controlled or limited space heater which limits amaximum temperature of the heater so that the generated heat isgenerally insufficient to cause overheating of the heater and the heatgenerated inhibits condensation on the windings during periods when themotor windings are not energized and the motor is not operating.

It is another object of this invention to provide a motor having athermostatically controlled or limited space heater which inhibitsoverheating of both the heater and the motor windings.

It is yet another object of this invention to provide a motor having athermostatically controlled or limited space heater wherein thethermostat prevents energization of the heater when the motortemperature is at or near a maximum desired operating temperature of themotor so that overheating of the motor and heater are prevented.

It is another object of this invention to provide a motor having athermostatically controlled or limited space heater in which no thermalinsulation is needed between the heater and the windings therebyenhancing heat transfer between the heater and the motor windings.

It is yet another object of this invention to provide a thermostaticallycontrolled or limited space heater for a motor which generates heatwhich does not raise the thermostat temperature above an open-circuittemperature of the thermostat during periods when the motor windings arenot energized and substantially all residual heat has dissipated fromthe motor thereby minimizing cycling of the thermostat during suchperiods.

It is another object of this invention to provide a thermostaticallycontrolled or limited space heater which is substantially continuouslyenergized during periods when the motor windings are not energized andsubstantially all residual heat has dissipated from the motor tosubstantially maintain the temperature of the windings above a sum ofthe predetermined minimum desired temperature rise plus an ambienttemperature.

An apparatus according to the invention is adapted to be connected to apower source and drive a rotatable component. A motor has a stationaryassembly and a rotatable assembly in magnetic coupling relation to thestationary assembly. The rotatable assembly is in driving relation tothe rotatable component. The stationary assembly includes windingsadapted to be energized by the power source to produce anelectromagnetic field for rotating the rotatable assembly, the windingshaving a maximum desired operating temperature and a predeterminedminimum desired temperature rise. A heater in heat exchange relationshipto an outer surface of the winding is adapted to be connected to thepower source at least when the motor windings are not energized togenerate heat transferred to the windings to increase the temperature ofthe windings. A thermostat connected in series between the heater andthe power source has an opening temperature and a closing temperaturesuch that when the temperature of the thermostat is below the closingtemperature, the thermostat is adapted to provide a closed circuitbetween the heater and the power source to energize the heater and suchthat when the temperature of the thermostat is above the temperature,the thermostat is adapted to provide an open circuit between the heaterand the power source to inhibit energizing the heater.

In one embodiment of the invention, the opening temperature of thethermostat is less than the maximum desired operating temperature of themotor windings so that the thermostat presents an open circuit and theheater does not generate heat immediately after energization of themotor windings is discontinued when the motor has reached the maximumdesired operating temperature. In another embodiment of the invention,the closing temperature of the thermostat is greater than an ambienttemperature of the motor so that the thermostat presents a closedcircuit and the heater generates heat when the thermostat temperature isbelow the closing temperature whereby the thermostat limits a maximumtemperature of the heater so that the generated heat is generallyinsufficient to cause overheating of the heater and the heat generatedinhibits condensation on the windings during periods when the motorwindings are not energized.

In one embodiment of the invention, the opening temperature is such thatthe thermostat provides a closed circuit resulting in the heater beingsubstantially continuously energized during periods when the motorwindings are not energized and substantially all residual heat hasdissipated from the motor to substantially maintain the temperature ofthe windings above a sum of the predetermined minimum desiredtemperature rise plus an ambient temperature. In another embodiment ofthe invention, the closing temperature of the thermostat is greater thanthe ambient temperature of the motor so that the thermostat presents aclosed circuit and the heater generates heat when the thermostattemperature is below the closing temperature whereby the heatergenerates heat which inhibits condensation on the windings duringperiods when the motor windings are not energized.

In one embodiment of the invention, the heater has a surface area and apower density such that the heat generated by the heater generally doesnot raise the temperature of the heater above the maximum temperatureduring periods when the motor windings are not energized whereby nothermal insulation is located between the heater and the windingsthereby enhancing heat transfer therebetween such that the heatgenerated by the heater does not cause the temperature of the motor orheater to exceed the maximum temperature and the heat generated by theheater inhibits condensation on the windings during periods when themotor windings are not energized.

The invention also includes a method of manufacturing a motor comprisingthe steps of:

providing a motor having a stationary assembly and a rotatable assemblyin magnetic coupling relation to the stationary assembly, the rotatableassembly in driving relation to a rotatable component, the stationaryassembly including windings adapted to be energized by a power source toproduce an electromagnetic field for rotating the rotatable assembly,the windings having a maximum desired operating temperature and apredetermined minimum desired temperature rise;

mounting a heater in heat exchange relationship to an outer surface ofthe windings, the heater adapted to be connected to the power source atleast when the motor windings are not energized to generate heattransferred to the windings to increase the temperature of the windings;

connecting a thermostat in series between the heater and a power source,the thermostat having an opening temperature such that when thetemperature of the thermostat is below the opening temperature, thethermostat is adapted to provide a closed circuit between the heater andthe power source to energize the heater and such that when thetemperature of the thermostat is above the opening temperature, thethermostat is adapted to provide an open circuit between the heater andthe power source to inhibit energizing the heater;

selecting the opening temperature such that the opening temperature ofthe thermostat is less than the maximum desired operating temperature ofthe motor windings so that the thermostat presents an open circuit andthe heater does not generate heat immediately after energization of themotor windings is discontinued when the motor has reached the maximumdesired operating temperature; and

selecting the closing temperature such that the closing temperature ofthe thermostat is greater than an ambient temperature of the motor sothat the thermostat presents a closed circuit and the heater generatesheat when the thermostat temperature is below the closing temperaturewhereby the thermostat limits a maximum temperature of the heater sothat the generated heat is generally insufficient to cause overheatingof the heater and the heat generated inhibits condensation on thewindings during periods when the motor windings are not energized.

The invention also includes a method of manufacturing a motor comprisingthe steps of:

providing a motor having a stationary assembly and a rotatable assemblyin magnetic coupling relation to the stationary assembly, the rotatableassembly in driving relation to a rotatable component, the stationaryassembly including windings adapted to be energized by a power source toproduce an electromagnetic field for rotating the rotatable assembly,the windings having a predetermined minimum desired temperature rise;

mounting a heater in heat exchange relationship to an outer surface ofthe windings, the heater adapted to be connected to the power source atleast when the motor windings are not energized to generate heattransferred to the windings to increase the temperature of the windings;

connecting a thermostat in series between the heater and a power source,the thermostat having an opening temperature and a closing temperaturesuch that when the temperature of the thermostat is below the closingtemperature, the thermostat is adapted to provide a closed circuitbetween the heater and the power source to energize the heater and suchthat when the temperature of the thermostat is above the openingtemperature, the thermostat is adapted to provide an open circuitbetween the heater and the power source to inhibit energizing theheater; and

selecting the closing temperature such that the thermostat provides aclosed circuit resulting in the heater being substantially continuouslyenergized during periods when the motor windings are not energized andsubstantially all residual heat has dissipated from the motor tosubstantially maintain the temperature of the windings above a sum ofthe predetermined minimum desired temperature rise plus an ambienttemperature whereby the heater generates heat which inhibitscondensation on the windings during periods when the motor windings arenot energized.

Other objects and features will be in part apparent and in part pointedout hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded, perspective view of a motor of the inventionincluding a strip heater on windings of the motor.

FIG. 2 is a partial plan view of a strip heater of the invention whichis mounted on the windings of the motor, the strip heater having athermostat on its surface.

FIG. 3 is a partial cross sectional view of a strip heater of theinvention directly on the end turns of the windings of a motor.

FIG. 4 is a diagrammatic illustration of a motor of the inventionincluding the motor and heater.

FIGS. 5, 6, 7 and 8 are graphs illustrating operation of: a motor of theprior art (FIG. 5); a motor of the invention having a heater of low wattdensity, increased surface area and no thermostat (FIG. 6); a motor ofthe invention having a thermostat (FIG. 7); and a motor of the inventionhaving both a thermostat and a heater of low watt density and increasedsurface area (FIG. 8). In each graph, time is along the X-axis andtemperature is along the Y-axis. These graphs are not to scale.

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, an exploded, perspective view of a motor 100 of theinvention is illustrated. Although this motor 100 is illustrated as anindustrial motor, it is contemplated that any motor having windingswhich must be heated is included in the invention. Motor 100 includes anend shield 102 which engages one end of frame and stator assembly 104.Assembly 104 constitutes a stationary assembly for receiving a rotatableassembly 106 in magnetic coupling relation to the assembly 104. Therotatable assembly 106 includes a shaft 108 which is in driving relationto a rotatable component (not shown). The frame and stator assembly 104includes windings 110 adapted to be energized by a power source 112 vialines 114 to produce an electromagnetic field for rotating the rotatableassembly 106. A second end shield 116 engages the other end of theassembly 104. End shields 102 and 116 include bearings or otherretainers known in the art for supporting the rotatable assembly 106within the frame and stator assembly 104. Optionally, the motor 100 mayinclude an external fan 118 engaging the shaft 108 for moving air aboutthe motor and assisting in its cooling. Also optional is a fan cover120.

The motor windings 110 tend to heat up during operation of the motorbecause of the current flowing through the windings 110 as provided bythe power source 112 via lines 114. When the motor is not operating, theresidual heat which has built up in the motor slowly dissipates untilthe motor reaches a temperature equal to the ambient temperature of theair or structure around the motor. In general, the residual heat is heatwhich has been added to the motor as a result of its operation. Duringdissipation of such residual heat and cooling of the motor, condensationcan develop on the motor windings which is undesirable. The condensationcan deteriorate the motor components, causing rusting orshort-circuiting of the motor and its components. As a result, eachmotor winding 110 has a predetermined minimum desired temperature abovethe ambient temperature. As long as the motor windings 110 aremaintained at or above the predetermined minimum desired temperature,the collection of condensation on the windings is inhibited orprevented. The difference between the predetermined minimum desiredtemperature and the ambient temperature is referred to as apredetermined minimum desired temperature rise.

A heater 122 in heat exchange relationship to an outer surface of thewindings 110 is positioned on the windings. The heater 122 is adapted tobe connected to the power source 112 at least during periods when themotor windings 110 are not energized. The heater 122 generates heattransferred to the windings to increase the temperature of the windings110 to avoid condensation when the motor 100 cools.

The heater 122, which is illustrated in more detail in FIG. 2, ispreferably a flexible strip heater having a resistive element body 124through which current flows and is converted into heat. The body 124 isconnected to terminals 126 and 128 for supplying electrical current tothe body 124. Preferably, an optional thermostat 130 is mounted on thesurface of the body 124 of heater 122 and is connected in series betweenthe body 124 and terminal 128. The thermostat has an opening temperatureand a closing temperature causing the thermostat to operate in thefollowing manner. When the temperature of the thermostat 130 is belowthe closing temperature, the thermostat is adapted to provide a closedcircuit between the heater and the power source to energize the heater122 (see FIG. 4). When the temperature of the thermostat 130 is abovethe opening temperature, the thermostat is adapted to provide an opencircuit between the heater 122 and the power source 112 to inhibitenergizing of the heater 122.

Depending on the motor configuration, its location, application, andenvironment, the motor has a maximum desired operating temperature whichshould not be exceeded. In some environments, this maximum desiredoperating temperature may equal the steady state operating temperatureof the motor. In order to prevent overheating of the motor 100 and/orthe heater 122, the opening temperature of the thermostat 130 is lessthan the maximum desired operating temperature. For example, thethermostat presents an open circuit and the heater does not generateheat immediately after energization of the motor windings isdiscontinued when the motor has reached a steady state operatingtemperature. In addition, the closing temperature of the thermostat 130is greater than the ambient temperature of the motor 100 so that thethermostat 130 presents a closed circuit and the heater 122 generatesheat when the thermostat temperature is below the closing temperature.As a result, thermostat 130 limits a maximum temperature of the heater122, motor windings 110, and motor 104 so that the heat generated by theheater 122 is generally insufficient to cause overheating of the heater,motor or motor windings. At the same time, the heat generated by theheater 122 inhibits condensation on the windings during periods when themotor windings are not energized.

As illustrated in more detail in FIG. 3, it is contemplated that thewindings 110 may include end turns 132 and it is further contemplatedthat the heater 124 be in direct contact with the end turns 132 of thewindings 110. In other words, the body 124 of the heater 122 is adhereddirectly to the outer layer of windings to enhance heat transfer betweenthe heater 122 and the motor windings 110. It is also contemplated thatno thermal insulation would be positioned between the heater 122 and thewindings 110, other than the electrical insulative coating on the wiresof windings 110.

Furthermore, it is contemplated that the thermostat 130 be located on asurface 134 of the body 124 of the heater 122. This location allows thethermostat to accurately monitor the temperature of the heater therebypreventing overheating of the heater. The closing temperature of thethermostat 130 is preferably at or above the sum of the predeterminedminimum desired temperature rise of the motor windings 110 plus theambient temperature. The closing temperature allows maintaining awinding temperature rise above ambient to prevent condensation. Theopening temperature of the thermostat 130 is preferably at or below themaximum desired operating temperature. In this way, the open circuitcaused by the thermostat when its temperature equals the openingtemperature inhibits overheating of the heater and the motor windings.In one preferred embodiment of the invention, the predetermined minimumdesired temperature rise is equal to a temperature rise of the windingscaused by continuous operation of the heater in a steady state conditionafter substantially all residual heat has been dissipated from themotor.

There are four factors that determine the effectiveness of thethermostatically controlled or limited space heater according to theinvention. The first factor in determining effectiveness is that thethermostat must be properly located and selected to have the appropriateopening and closing temperatures. As noted above, the thermostat 130 ispreferably mounted on the surface 134 of the heater 122 and connected inseries between the heater 122 and the power source 112. The openingtemperature of the thermostat 130 is selected in order to limit themaximum surface temperature of the heater surface 134. Depending on thetype of thermostat mounting, temperature variations over the heatersurface, and the heat transfer variation, the opening temperature of thethermostat 130 may differ from the maximum desired temperature.Preferably, a hermetically sealed thermostat may be used to comply withstandards that require devices to be arcless and sparkless.

The second factor in determining effectiveness is the surface area ofthe heater 122. Preferably, the heater has a large surface area between15-225% greater than the standard surface area. This large surface areagenerates the power needed to efficiently heat the windings 110 anddistribute heat throughout the windings sufficient to preventcondensation.

The third factor in determining effectiveness is the power density ofthe heater 122. Preferably, the heater 122 has a low power density of1.0 watts/in² or less, which minimizes the temperature variationsoccurring over the heater surface. The low power density also reducessurface temperatures and allows the heater 122 to be directly mounted onthe motor winding end turns 132 (see FIG. 3), which improves the rate ofheat transfer from the heater 122 to the windings 110.

The following Tables 1 and 2 illustrate a comparison of various sizedheaters having standard surface area and power density of the prior artas contrasted with the large surface area and low power density of theinvention:

                  TABLE 1                                                         ______________________________________                                        Comparison of Surface Area                                                    of Prior Art Heaters and Heaters of the Invention                                                            LARGE SURFACE                                                  STANDARD SURFACE                                                                             AREA OF                                        HORSE- FRAME    AREA OF PRIOR ART                                                                            INVENTION                                      POWER  SIZE     (in.sup.2)     (in.sup.2)                                     ______________________________________                                        1-5    180      9.0            13.5                                           2-30   210-280  30.0           35.0                                           25-125 320-400  49.0           72.4                                           100-350                                                                              440      70.0           152.8                                          200-1500                                                                             500      94.5           213.4                                          ______________________________________                                    

                                      TABLE 2                                     __________________________________________________________________________    Comparison of Power and Power Density                                         of Prior Art Heaters and Heaters of the Invention                                       STANDARD                                                                             LOW POWER                                                                            STANDARD LOW POWER                                              POWER OF                                                                             OF     POWER DENSITY                                                                          DENSITY OF                                   HORSE-                                                                             FRAME                                                                              PRIOR ART                                                                            INVENTION                                                                            OF PRIOR ART                                                                           INVENTION                                    POWER                                                                              SIZE (watts)                                                                              (watts)                                                                              (watts/in.sup.2)                                                                       (watts/in.sup.2)                             __________________________________________________________________________    1-5  180  45     13     5.0      1.0                                          2-30 210-280                                                                            60     35     2.0      1.0                                          25-125                                                                             320-400                                                                            100    70     2.0      1.0                                          100-350                                                                            440  145    125    2.1      0.8                                          200-1500                                                                           500  350    200    3.7      0.9                                          __________________________________________________________________________

Tables 1 and 2 are based on known non-optimized operation of existingmotors and heaters. In general, the surface area of the invention isabout twice the standard surface area and the power of the invention isabout half the standard power so that the power density of the inventionis about 25% of the standard power density (power density=power/surfacearea). This is, in part, based on known non-optimized prior art. Theactual surface area and power are functions of thermal attachment ofheater to windings, motor dissipation rate and stator/motor dimensions(which also affects dissipation rate).

A fourth factor which determines effectiveness of the heater 122 is theoperating duty cycle during normal operation. The combination of thethermostat 130 with its opening and closing temperatures, larger surfacearea, and lower power density allows the space heaters of the inventionto operate continuously under normal conditions. Continuous operationallows the space heater 122 to provide a constant temperature rise inthe motor winding which will prevent condensation. In this situation,one function of the thermostat is to limit excess temperature. Properselection of the opening and closing temperatures, surface area, andpower density will minimize thermostat cycling which will provide longerlife for the thermostat. Continuous operation means that the thermostatis not cycling and switching from on and off conditions. In systemswhich do not employ a thermostat, the first factor does not apply andthe remaining factors become more significant.

Referring to FIGS. 5-8, graphs are presented illustrating operation ofvarious motors. FIG. 5 illustrates the operation of a motor of the priorart with time along the X-axis and temperature of the motor or motorwindings along the Y-axis. The graph of FIG. 5 assumes that the priorart motor has a space heater which is constantly energized duringperiods of motor shutdown. At time zero, it is assumed that the motor isshut down after reaching a steady state operating temperature of 140° C.According to FIG. 5, this prior art motor has a winding temperature risedue to operation of 100° C. since it is assumed that the ambienttemperature is 40° C. The graph also assumes a heater temperature riseof 80° C. At time zero, the motor is shut down and the heater isimmediately energized which causes the temperature of the heater surfaceto rise from 140° C. to 220° C., as illustrated by line HS (heatersurface). This causes the motor windings to increase in temperature from140° C. to 145° C., as indicated by line MWH (motor winding withheater). As the motor cools and residual heat of the motor dissipates,the motor reaches a steady state static condition at time T. At thattime, the motor winding temperature is approximately 45° C. as indicatedby line MWH and the heater surface temperature is approximately 120° C.as indicated by line HS. Line MW (motor winding without heater) has beenadded to FIG. 5 to illustrate the temperature of the motor windings ifthe heater is not operated. At time T and thereafter, it can be seenthat a temperature rise of approximately 5° C. of the motor windingsresults from heater operation to prevent or inhibit condensation. It canalso be seen that at time T and thereafter there is a temperaturedifference of approximately 75° C. between the temperature of the motorwindings and the temperature of the heater surface. This is due in partto thermal insulation between the heater and the windings andtemperature variations on the heater. This large differential betweenthe temperature of the motor windings and the temperature of the heatersurface is undesirable, particularly in hazardous installations. Also,heaters of the prior art generally have high watt densities which createwide temperature ranges between hot spots and cold spots on the heater.

FIG. 6 illustrates a motor of the invention having a heater with lowerwatt density than the heater of FIG. 5, with a larger surface area thanthe heater of FIG. 5 and with no thermostat. The lower watt densitymeans that the heater surface HS peaks at 180° C. rather than 220° C. asillustrated in FIG. 5 providing a heater temperature rise of 40° C. Onthe other hand, the increased surface area of the heater allows theheater to raise the motor winding temperature by 5° C., the same amountas illustrated in FIG. 5. In this embodiment, some thermal insulationmay be required to prevent overheating of the windings immediately aftershutdown. If a momentary over-temperature condition of the windings isacceptable, the heater may be installed directly on the winding.Condensation is again prevented by maintaining the winding temperature5° C. above ambient. The low watt density reduces temperature extremesbetween the hot and cold spots on the heater. When the heater is mounteddirectly on the windings, the wattage may be reduced further because ofbetter heat transfer.

Referring again to FIG. 6, it can be seen that the heater is providedwith a surface area and a power density such that the heat generated bythe heater generally does not raise the temperature of the heater abovethe maximum temperature of 180° C. (which is lower than the maximum of220° C. of FIG. 5) during periods when the motor windings are notenergized. As illustrated in FIG. 6, it can be seen that the temperatureof the heater surface HS after time T is below the steady state motorrunning temperature of 140° C. This is because the heat generated by theheater does not cause the temperature of the heater or motor windings orthe motor to exceed the maximum temperature. At the same time, the heatgenerated by the heater inhibits condensation on the windings duringperiods when the motor windings are not energized by causing a 5°temperature rise in the motor windings to prevent condensation.

FIG. 7 illustrates a motor according to the invention including a spaceheater having a thermostat mounted on the heater. Regarding FIG. 7, ithas been assumed that the heater does not have a low watt density or alarge surface area. For example, FIG. 7 would illustrate the motor ofFIG. 5 including a thermostat on the heater. FIG. 7 assumes that thethermostat has a closing temperature of 80° C. and an openingtemperature of 90° C. At time zero, the motor is shut down afteroperating and reaching the motor maximum or operating temperature of140° C. At time zero, it is assumed that the heater and the thermostatare at the same temperature as the motor. Therefore, due to the openingand closing temperatures of the thermostat, the thermostat is in an opencircuit condition and the heater is not energized. FIG. 7 assumes thatthe heater, thermostat and motor windings all cool at the same rate andat the same temperature as residual heat is dissipated. At time Q, theheater, thermostat and motor windings reach a temperature of 80° C.which is the closing temperature of the thermostat. Thereafter, theheater is energized because the thermostat presents a closed circuitbetween the power supply and the heater causing the heater surfacetemperature HS to rise. At time R the heater surface temperature isapproximately 140° C. It is assumed that the heater surface temperatureincreases more quickly than the temperature of the thermostat because ofdelay in heat transfer from the heater to the thermostat. FIG. 7 assumesthat a time R or slightly before time R, the heater surface has atemperature of 140° C. and the thermostat has a temperature of 90° C.Since this is the opening temperature of the thermostat, the thermostatpresents an open circuit to the heater and further energization of theheater is discontinued. After time R, the heater surface begins toslowly cool as indicated by line HS. As this cooling occurs, it isassumed that the thermostat and the heater equalize and becomeapproximately the same temperature. The heater surface and thermostatcontinue to cool until time S when they reach a temperature of 80° C.,which is the closing temperature. At this point, the thermostat cyclesagain. As a result, the thermostat can be used to limit the maximumheater surface temperature to be less than the motor operatingtemperature of 140° C. However, temperature variations on the heaterbetween the hot and cold spots can make such control of the maximumtemperature difficult. Also, the thermostat will cycle on and offcontinuously to produce required wattage and limit surface temperature.Over time, this cycling may reduce the life of the thermostat. FIG. 7assumes that sufficient heat is produced by each cycle of the thermostatto maintain adequate heat generation to prevent condensation. In certainsituations, it may be necessarily desirable to increase the openingtemperature or decrease the closing temperature to maintain adequateheat generation to prevent condensation. However, the openingtemperature cannot be increased to the point that one cycle of thethermostat results in the heater surface temperature exceeding themaximum temperature. The duty cycle of the thermostat of FIG. 7 isillustrated as relatively large for convenience. In fact, the thermostatmay cycle frequently (once per minute or more often) in order to limitthe temperature and produce sufficient heat to maintain the motorwinding temperature above ambient.

FIG. 8 illustrates one preferred embodiment of the invention accordingto FIGS. 1-4 including a low watt density, large surface area heaterwith a thermostat mounted thereon. In this illustration, the thermostatis assumed to have a closing temperature of 80° C. and an openingtemperature of 90° C. The same basic assumptions regarding theillustration of FIG. 7 have been assumed for FIG. 8. At time zero, themotor is shut down. The motor windings, heater and thermostat are at thesame temperature and cool as the residual heat added to the motor due toits operation continues to dissipate over time. At time Q, thetemperature of the thermostat, heater and motor winding cools to 80° C.which is the closing temperature thereby causing the thermostat topresent a closed circuit between the heater and the power supply. Thisenergizes the heater and causes its surface temperature to increase asindicated by line HS. By assumption, the heater temperature increasesmore quickly than the thermostat temperature so that the heatertemperature has reached the peak of about 110° C. when the thermostattemperature reaches its opening temperature of 90° C. and presents anopen circuit to deenergize the heater. This deenergization occurs attime R. Thereafter, the heater and thermostat cool at approximately thesame temperature until time S when the thermostat reaches its closingtemperature of 80° C. and the cycle repeats itself. One differencebetween FIGS. 7 and 8 is the magnitude of successive peaks of the cyclesof the thermostat. In FIG. 7, the peaks are approximately equal and aredesigned to occur at or near the maximum temperature of 140° C. In FIG.8, the lower watt density and larger surface area of the heater meansthat the heater will heat the motor windings more slowly and evenly sothat successive peaks will be reduced as the motor winding temperaturecontinues to decline. Eventually, at time T, the motor windings, heaterand thermostat stabilize to the point where the dissipating heat equalsthe heat generated by the heater thereby maintaining the motor windingtemperature MWH at 5° C. above ambient. Between times Q and T thethermostat cycles. After time T, the thermostat remains a closed circuitand does not cycle until the next motor operating cycle.

As illustrated in FIGS. 7 and 8, the thermostat limits the maximumsurface temperature of the heater as indicated by line HS so that thegenerated heat is generally insufficient to cause overheating of theheater. In other words, the surface temperature of the heater neverexceeds the maximum desired operating temperature of the motor of 140°C., which may also be the steady state operating temperature of themotor. On the other hand, the thermostat permits sufficient heatgeneration to inhibit condensation on the windings during periods whenthe motor windings are not energized. Also, it can be seen that theclosing temperature of the thermostat is at or above a sum of thepredetermined minimum desired temperature rise of the windings plus theambient temperature. For FIGS. 7 and 8, the closing temperature of 80°C. is above 45° C. As a result, the opening temperature inhibitsoverheating of both the heater and the motor windings. It can also beseen that the opening temperature of the thermostat is less than themaximum desired operating temperature of the motor minus a motortemperature rise caused by the heater when continuously energized. As aresult, the thermostat presents an open circuit and the heater does notgenerate heat when the motor or its windings is at or near the maximumdesired operating temperature of the motor. Once again, the thermostatthus prevents energization of the heater when the motor temperature isat or near the maximum desired operating temperature so that overheatingof the motor and heater are prevented.

As shown in FIG. 8, the minimum predetermined temperature rise is equalto a temperature rise of the windings caused by continuous operation ofthe heater in a steady state condition after substantially all residualheat has been dissipated from the motor, i.e., after time T. FIG. 8 alsoillustrates that the heater is continuously energized and the thermostatis in the closed condition during periods when the motor windings arenot energized and substantially all residual heat has dissipated fromthe motor. As shown in FIGS. 1 and 4, the heater 122 may be continuouslyconnected to the power supply 112 via the thermostat 130. However, it isalso contemplated that the power source 112 include a shunting circuitwhich continuously connects the heater 122 to the power supply 112 viathe thermostat 130 only during periods when the motor windings 110 arenot energized. In other words, either the motor windings 110 or theheater 122 is energized but both are not energized simultaneously.Comparing FIGS. 7 and 8, it can be seen that the motor of FIG. 8maintains a maximum heater temperature (peaks of HS) which is lower thanthe maximum heater temperature of FIG. 7. As a result, the motor of FIG.8 can accommodate hot and cold spots on the heater with reasonableassurance that the heater temperature will not exceed the maximum orsteady state operating temperature of the motor. Therefore, the motor ofFIG. 8 is particularly suited for hazardous environments.

The invention also includes a method of manufacturing a motor as notedabove. In particular, a motor 100 is provided with a heater 122 mountedin heat exchange relationship to the outer surface of the windings 110.The thermostat 130 is connected in series between the heater 122 and thepower source 112. The opening temperature of the thermostat 130 isselected to be less than the maximum desired operating temperature ofthe motor windings so that the thermostat presents an open circuit andthe heater does not generate heat immediately after energization of themotor windings is discontinued when the motor has reached the maximumdesired operating temperature, i.e., time zero of the graphs. Theclosing temperature is selected to be greater than the ambienttemperature of the motor so that the thermostat presents a closedcircuit and the heater generates heat when the thermostat temperature isbelow the closing temperature, e.g., between time Q and time R. As aresult, the thermostat limits the maximum temperature of the heater sothat the generated heat is generally insufficient to cause overheatingof the heater and the heat generated inhibits condensation on thewindings during periods when the motor windings are not energized.

As noted above, the method of manufacture may include mounting thethermostat on the surface of the heater. In addition, the closingtemperature of the thermostat may be at or above a sum of thepredetermined minimum desired temperature rise of the windings plus theambient temperature. As a result, the opening temperature inhibitsoverheating of the heater and the motor windings.

As noted above, the surface area, power density, and closing temperatureof the thermostat may be selected so that the heat generated by theheater generally does not raise the thermostat temperature above theopening temperature during periods when the motor windings are notenergized and substantially all residual heat has dissipated from themotor thereby minimizing cycling of the thermostat during such periods.This aspect is best illustrated by comparing FIGS. 7 and 8. In FIG. 7,the thermostat will cycle until the motor is energized. Whereas, in FIG.8, after time T, the thermostat is in a continuously closed circuitcondition and does not cycle.

The method may also include mounting the heater in direct contact withthe end turns of the windings so that no thermal insulation is locatedtherebetween thereby enhancing heat transfer between the heater and themotor windings.

FIG. 8 illustrates the results of selecting the opening and closingtemperatures such that the heater is continuously energized and thethermostat provides a closed circuit during periods when the motorwindings are not energized and substantially all residual heat hasdissipated from the motor, i.e., after time T.

In the embodiment illustrated in FIG. 6, the heater has a surface areaand a power density such that the heat generated by the heater generallydoes not raise the temperature of the heater above the maximumtemperature during periods when the motor windings are not energized andsubstantially all residual heat has dissipated from the motor, i.e.,after time T. As a result, the heat generated by the heater does notcause the temperature of the motor to exceed the maximum temperature andthe heat generated by the heater inhibits condensation on the windingsduring periods when the motor windings are not energized, i.e., aftertime T.

The maximum surface temperature of the heater, when mounted directly onthe end turns, will be the sum of the ambient temperature, the windingtemperature rise due to operation of the motor, and the heatertemperature rise due to energization of the heater. If the sum of theambient temperature and the winding temperature rise exceeds 140° C.(the maximum desired operating temperature), the thermostat will notallow the heater to operate, but the heater surface temperature will beequal to the total temperature of the winding, as shown in FIGS. 7 and8.

In general, thermostat operation is transient depending upon heat outputof the heater, thermal resistance between the heater and the thermostatand the relative rate of the cooling of the motor. Thus, the thermostatopening temperature, as illustrated in FIGS. 7 and 8 does not exactlycorrespond to the maximum temperature of the heater. Furthermore, FIGS.5, 6 and 7 are based on assumed optimization of existing motors andheaters.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above without departing from thescope of the invention, it is intended that all matter contained in theabove description and shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

What is claimed is:
 1. A method of manufacturing a motor comprising thesteps of:providing a motor having a stationary assembly and a rotatableassembly in magnetic coupling relation to the stationary assembly, therotatable assembly in driving relation to a rotatable component, thestationary assembly including windings adapted to be energized by apower source to produce an electromagnetic field for rotating therotatable assembly, the windings having a maximum desired operatingtemperature and a predetermined minimum desired temperature rise;mounting a heater having a heating element in a conductive heat exchangerelationship to and on an outer surface of the windings, the heateradapted to be connected to the power source at least when the motorwindings are not energized to generate heat transferred from the heatingelement to the windings to increase the temperature of the windings;connecting a thermostat for sensing an operating temperature of theheater and for sensing an operating temperature of the motor windings inseries between the heater and a power source, the thermostat having anopening temperature such that when the temperature of the thermostat isbelow the opening temperature, the thermostat is adapted to provide aclosed circuit between the heater and the power source to energize theheater and such that when the temperature of the thermostat is above theopening temperature, the thermostat is adapted to provide an opencircuit between the heater and the power source to inhibit energizingthe heater; selecting the opening temperature such that the openingtemperature of the thermostat is less than the maximum desired operatingtemperature of the motor windings so that the thermostat presents anopen circuit and the heater does not generate heat immediately afterenergization of the motor windings is discontinued when the motor hasreached the maximum desired operating temperature; and selecting theclosing temperature such that the closing temperature of the thermostatis greater than an ambient temperature of the motor so that thethermostat presents a closed circuit and the heater generates heat whenthe thermostat temperature is below the closing temperature whereby thethermostat limits a maximum temperature of the heater so that thegenerated heat is generally insufficient to cause overheating of theheater and the heat generated inhibits condensation on the windingsduring periods when the motor windings are not energized.
 2. The methodof claim 1 further comprising the step of mounting the thermostat on asurface of the heater and wherein the closing temperature of thethermostat is at or above a sum of the predetermined minimum desiredtemperature rise of the windings plus the ambient temperature wherebythe thermostat inhibits overheating of the heater and the motorwindings.
 3. The method of claim 2 further comprising the steps ofselecting the heater with a surface area and a power density andselecting the closing temperature of the thermostat such that the heatgenerated by the heater generally does not raise the thermostattemperature above the closing temperature during periods when the motorwindings are not energized and substantially all residual heat hasdissipated from the motor thereby minimizing cycling of the thermostatduring such periods.
 4. The method of claim 3 further comprising thestep of selecting the surface area, the power density and the openingtemperature of the thermostat such that the generated heat is generallyinsufficient to cause heating of the heater above the maximum desiredoperating temperature of the motor whereby the thermostat limits amaximum temperature of the heater so that the generated heat isgenerally insufficient to cause overheating of the heater.
 5. The methodof claim 4 wherein the windings include end turns and further comprisingthe step of mounting the heater in direct contact with the end turns ofthe windings whereby no thermal insulation is located between the heaterand the windings thereby enhancing heat transfer between the heater andthe motor windings.
 6. The method of claim 2 further comprising the stepof selecting the heater with a surface area and a power density andselecting the opening temperature of the thermostat such that thegenerated heat is generally insufficient to cause heating of the heaterabove the maximum desired operating temperature of the motor whereby thethermostat limits a maximum temperature of the heater so that thegenerated heat is generally insufficient to cause overheating of theheater.
 7. The method of claim 2 further comprising the step ofselecting the opening temperature of the thermostat to be less than themaximum desired operating temperature of the motor minus a motortemperature rise caused by the heater when continuously energized sothat the thermostat presents an open circuit and the heater does notgenerate heat when the motor is at or near the maximum desired operatingtemperature whereby the thermostat prevents energization of the heaterwhen the motor temperature is at or near the maximum desired operatingtemperature so that overheating of the motor and heater are prevented.8. The method of claim 1 wherein the predetermined minimum desiredtemperature rise is equal to a temperature rise of the windings causedby continuous operation of the heater in a steady state condition aftersubstantially all residual heat has been dissipated from the motor. 9.The method of claim 1 wherein the windings include end turns and furthercomprising the step of mounting the heater in direct contact with theend turns of the windings whereby no thermal insulation is locatedbetween the heater and the windings thereby enhancing heat transferbetween the heater and the motor windings.
 10. The method of claim 1further comprising the step of selecting the thermostat to provide aclosed circuit during periods when the motor windings are not energizedsubstantially all residual heat has dissipated from the motor.
 11. Themethod of claim 1 further comprising the step of continuously connectingthe heater to the power supply via the thermostat.
 12. The method ofclaim 1 further comprising the step of continuously connecting theheater to the power supply via the thermostat only during periods whenthe motor windings are not energized.
 13. The method of claim 6 whereinthe power density is an order of 1.0 watts/in².
 14. A method ofmanufacturing a motor comprising the steps of:providing a motor having astationary assembly and a rotatable assembly in magnetic couplingrelation to the stationary assembly, the rotatable assembly in drivingrelation to a rotatable component, the stationary assembly includingwindings adapted to be energized by a power source to produce anelectromagnetic field for rotating the rotatable assembly, the windingshaving a maximum desired operating temperature and a predeterminedminimum desired temperature rise; mounting a heater having a heatingelement in a conductive heat exchange relationship to and on an outersurface of the windings, the heater adapted to be connected to the powersource at least when the motor windings are not energized to generateheat transferred from the heating elements to the windings to increasethe temperature of the windings; connecting a thermostat for sensing anoperating temperature of the heater and for sensing an operatingtemperature of the motor windings in series between the heater and apower source, the thermostat having an opening temperature and a closingtemperature such that when the temperature of the thermostat is belowthe closing temperature, the thermostat is adapted to provide an closedcircuit between the heater and the power source to energize the heaterand such that when the temperature of the thermostat is above theopening temperature, the thermostat is adapted to provide an opencircuit between the heater and the power source to inhibit energizingthe heater; and selecting the closing temperature such that thethermostat provides a closed circuit resulting in the heater beingsubstantially continuously energized during periods when the motorwindings are not energized and substantially all residual heat hasdissipated from the motor to substantially maintain the temperature ofthe windings above a sum of the predetermined minimum desiredtemperature rise plus an ambient temperature whereby the heatergenerates heat which inhibits condensation on the windings duringperiods when the motor windings are not energized.
 15. The method ofclaim 14 further comprising the step of selecting the openingtemperature of the thermostat to be less than the maximum desiredoperating temperature of the motor so that the thermostat presents anopen circuit and the heater does not generate heat immediately afterenergization of the motor windings is discontinued when the motor hasreached a desired operating temperature whereby the thermostat limits amaximum temperature of the heater so that the generated heat isgenerally insufficient to cause overheating of the heater.
 16. Themethod of claim 15 further comprising the step of applying the heater indirect contact with the end turns of the windings whereby no thermalinsulation is located between the heater and the windings therebyenhancing heat transfer between the heater and the motor windings.